The present invention relates to improvements in and relating to investment casting. In particular, the invention relates to a method of investment casting which involves the successive application of one or more coats of a refractory slurry to an expendable pattern, so as to build up a shell. The present invention further provides a refractory slurry for use in the method of the invention, and a kit of ingredients for putting the method of the invention into effect.
The process of investment casting, otherwise known as the lost wax process, is well known and widely used. Typically, the process involves dipping a wax model into a slurry comprising a binder and a refractory material, so as to coat the model with a layer of slurry; applying a stucco coating of dry refractory to the surface of the layer; allowing the resulting stuccoed slurry layer to dry; and applying further stuccoed slurry layers as appropriate to create a shell mould around the wax model having a suitable thickness. After thorough drying, the wax model is eliminated from the shell mould, and the mould is fired.
Environmental considerations dictate that the binder used in the investment casting process should be water-based, rather than alcohol-based. Customarily, the binder used comprises an aqueous colloidal silica sol. When combined with a suitable refractory material into a slurry, aqueous silica sols are capable of gelling and drying to form a green shape having an acceptable degree of green strength. However, where an unmodified aqueous silica sol is used, the time taken for this process is disadvantageously long. A single stuccoed slurry layer, applied to a wax model in the course of investment casting, may take between 3-8 hours to dry. Where the model comprises recessed parts or other complex configurations, the drying time may be increased to 24 hours or more. During production of a shell mould having several stuccoed layers, this time must be multiplied by the number of coats applied. Typically, 4-8 coats are required in order to build a shell of acceptable thickness, thereby bringing the total production time to the order of between 12 hours and several days.
Considerable effort has therefore been devoted in the prior art towards accelerating the coat drying time. Thus, for example, EP-A-0638379 discloses that the addition to a colloidal silica sol binder of an elastomeric polymer, such as styrene butadiene, results in a significant reduction in the drying time and an improvement in green strength. The addition of soluble organic polymers to a colloidal silica sol binder for use in investment casting is also disclosed in U.S. Pat. No. 4,996,084. Soluble organic polymers however readily xe2x80x9cwet outxe2x80x9d, and it has been found that the green strength of a shell mould comprising such polymers is temporarily reduced by the permeation of steam through the mould, for example during the elimination of the wax model from the mould. Moreover, soluble organic polymers are expensive, and their use in investment casting may significantly increase the cost of this process.
It remains therefore a desirable object to provide an alternative means for reducing the time required to build a shell mould of suitable thickness during the process of investment casting.
According to one aspect of the present invention, therefore, there is provided a method of investment casting, comprising the steps of mixing a binder, a refractory material, and a quantity of water-insoluble organic fibres to form a slurry; coating an expendable pattern with a coat of said slurry; and drying said coat or allowing said coat to dry to form a shell.
Optionally, a plurality of coats may be applied successively to said expendable pattern, each coat being partly or wholly dried or allowed to dry prior to the application of the next coat. Typically, between 2-10 coats, more preferably 3-8 coats, still more preferably 3, 4 or 5 coats, are successively applied to said pattern. In some embodiments, said expendable pattern is precoated in accordance with known conventional methods with a coat of slurry comprising no water-insoluble organic fibres, prior to the application in accordance with the present invention of one or more coats of fibre-modified slurry.
According to a further aspect of the present invention, there is provided a slurry comprising a binder, a refractory material and a quantity of water-insoluble organic fibres, which slurry is adapted for use in the method of the present invention.
According to yet another aspect of the present invention, there is provided a kit adapted for putting the method of the present invention into effect, said kit comprising a quantity of a binder, a quantity of a refractory material, and a quantity of water-insoluble organic fibres; said binder, refractory material and fibres being adapted to be mixed to form a slurry in accordance with the present invention. Said refractory material may be packaged and/or supplied separately from the other ingredients of the kit.
Surprisingly, the present inventors have found that a refractory slurry comprising a quantity of water-insoluble organic fibres is capable of forming significantly thicker coats around dipped objects than are slurries of the types known in the prior art. An increase in coat thickness obviously implies a concomitant decrease in the number of dipping cycles required to build a mould of sufficient thickness, and hence a significant reduction in the rate of refractory mould production. Coats of fibre-modified slurry are subject to a comparable drying time in comparison with the products of the prior art, and have been found to possess a comparable green strength.
It has furthermore been found that the efficacy of elastomer-modified binders of the kind disclosed in EP-A-0638379 and U.S. Pat. No. 4,996,084 is largely destroyed by exposure of the binders to low temperatures (0xc2x0 C. and below). In contrast to the elastomer-modified compositions of the prior art, however, fibre-modified refractory slurries in accordance with the present invention have been found to be compatible with many types of antifreezes. This will therefore make possible the addition of antifreeze to binders intended for use in the fibre-modified slurries of the present invention, hence facilitating winter transport and storage of such binders.